1. Field of the Invention
The present invention relates to a double-sided wiring board fabrication method, a double-sided wiring board and a base material therefor and, particularly, to a double-sided wiring board fabrication method, a double-sided wiring board and a base material therefor, capable of forming a fine wiring even if a copper film electro deposited on exposed copper foil becomes thick during electroplating a metal layer for electrical conduction on the wall of a through-hole or blind via-hole.
2. Description of the Related Art
With the rapid flow of size reduction, high integration, and performance enhancement of electronic devices, double-sided wiring boards attract attention as boards for mounting IC chips. Double-sided wiring boards have a copper foil pattern layer on both upper and lower (front and back) sides of the board made of glass epoxy, polyimide, or the like, as an insulating layer, and through-holes or blind via-holes (via-holes having a bottom) formed at any positions of the front and back patterns, where the inner walls of the through-holes and blind via-holes are made electrically conductive by plating treatment or paste embedding (See Japanese patent No. 3593234, for example).
It has been confirmed that such double-sided wiring boards are advantageous in high-density packaging because of wiring patterns on both front and back sides thereof, and excellent in electrical properties as well.
FIGS. 1A-1F are cross-sectional views showing the respective steps of a conventional double-sided wiring board fabrication process using a glass epoxy board (through-hole processing).
In the step of making a double-sided wiring base material, a copper foil 11 is laminated on both sides of a glass epoxy board 1 (FIG. 1A). Taking handling into account, the copper foil 11 is generally not less than 18 μm thick.
Generally, the method for electrical conduction at via-holes in both upper and lower wiring (FIG. 1B) forms through-holes 5 at desired positions by means of a laser, punch, drill, or the like before etching processing for forming wiring in the copper foil 11; (FIG. 1C) plates the exposed hole walls with electroless copper to form an electroless copper plating film 6 for simplified electrical conduction; and subsequently (FIG. 1D) applies electrolytic copper plating to form an electrolytic copper plating film 7 for making the film thickness thick.
As the initial treatment for electrical conduction for the through-hole walls, the above-mentioned electroless copper plating treatment has many uses in glass epoxy boards, but palladium catalyst addition, conductive material coating, or the like may be used.
The opening diameter of the through-holes 5 is as fine as the minimum 100 μm in the case of punching or drilling. An electroless copper plating solution is caused to penetrate into these fine openings to form a thin copper coating film, and the upper and lower copper foil 11 is caused to serve as a cathode, followed by causing current to flow, allowing further thick copper plating to be electrodeposited on the through-hole 5 walls and the copper foil 11, which result in electrical conduction in the through-hole 5 formation portions of the copper foil 11 established on both upper and lower sides of the glass epoxy board 1 (insulative base material).
A double-sided wiring board is completed generally by the subsequent photoetching processing of the copper foil 11 into a finished fine wiring pattern 8 (FIG. 1E), to which is further applied a surface treatment film 9 (FIG. 1F) matching the purposes of mounting of electronic components, packaging on the print wiring board, bonding to the glass board, attaching to connector terminals, and so on.
FIGS. 2A-2H are cross-sectional views showing the respective steps of a conventional double-sided wiring board (flexible double-sided wiring board) fabrication process using a polyimide board (blind via-hole processing).
In the step of making a double-sided wiring base material, a copper foil 11 is laminated on both sides of a polyimide board 10 formed of a polyimide tape having a flexible property (FIG. 2A). For the flexible double-sided wiring board taking advantage of such a polyimide tape property, as thin a copper foil material as possible is used to make the copper foil 11 thin, but has a limit up to the order of 12 μm thickness as it stands.
The flexible double-sided wiring board requires a fine pitch at least in signal wiring particularly in order to enhance wiring density. For this reason, the holes for electrical conduction are generally non-through blind via-holes 12 (FIG. 2B).
The blind via-holes 12 are often formed by laser processing having many uses for fine wiring formation, because the via opening diameter is 40-80 μm, which is finer than the opening diameter formed by punching or drilling.
The treatment for electrical conduction within the blind via-holes 12 is substantially the same as in the case of the glass epoxy board 1 of FIG. 1, but a masking tape (insulating tape) 13 (FIG. 2C) is mounted on the copper foil surface opposite the via-hole opening surface to mask it so as not to make the film thick, because the thick copper foil film for signal wiring formation formed during copper plating of the via-holes causes extreme difficulty in etching formation of finer-pitch wiring.
Subsequently, a palladium catalyst 14 (FIG. 2D) is added to the inner walls of the blind via-holes 12 for simplified electrical conduction, followed by electrolytic copper plating to form an electrolytic copper plating film 7 (FIG. 2E) for making the film thickness thick.
As the initial treatment for electrical conduction for the blind via-hole walls, electroless copper plating treatment, conductive material coating, or the like may be used for or combined with the above-mentioned palladium catalyst addition.
A flexible double-sided wiring board is completed by subsequent masking tape 13 stripping (FIG. 2F) followed by photoetching processing of the copper foil 11 into a finished fine wiring pattern 8 (FIG. 2G), to which is further applied a surface treatment film 9 (FIG. 2H) matching the purposes of mounting of electronic components, packaging on the print wiring board, bonding to the glass board, attaching to connector terminals, and so on.
According to the conventional double-sided wiring boards, however, there is the problem of difficulty in fine wiring formation because the copper plating thickness for the electrically conductive treatment of the through-holes or blind via-holes is required to be as thick as more than 15 μm from the point of view of ensuring reliability in heat resistance and moisture resistance of the boards, in which case the copper foil thickness combined with the original thickness can be more than about 30 μm by applying copper plating to both the upper and lower copper foil surfaces in the case of the through-holes, and to the unmasked copper foil surface on the via-hole open side in the case of the blind via-holes with one of the upper and lower copper foil surfaces masked with the insulating tape or the like. In order to form fine wiring by etching, thinner copper foil thickness is advantageous. For instance, the wiring pitch limit which is possible to form by etching is 100 μm for 35 μm copper foil thickness, 80 μm for 25 μm copper foil thickness, and 60 μm for 18 μm copper foil thickness.
For the strong demand for the size-reduction of electronic components, the wiring pitch of wiring boards for mounting electronic components is increasingly narrower, which therefore requires making wiring still finer.